Kinetic aspects of ribosome assembly in HeLa cell nucleoli will continue to be emphasized. The various ribosomal precursor particles will be separated: their proteins will be analyzed by two-dimensional gel electrophoresis. The rate of appearance of newly synthesized protein of each type will be measured by double-label experiments, to determine the order of assembly of ribosomal proteins into ribosomes. Protein L5, which is associated with 5SrRNA in mature ribosomes, is the only ribosomal protein that has a large pool in the nucleolus; there are roughly thirteen molecules of protein L5 for every nascent 60S ribosomal subunit. We will continue to search for the function of the pool of L5, asking whether all is associated with 5S rRNA in the nucleoplasm, and so forth. Protein S2 has a half-life shorter than that of the ribosomal particles in the cytoplasm, but newly synthesized S2 cannot enter cytoplasmic ribosomes in the absence of ribosome synthesis (although three large subunit proteins can do so). We will try to determine the function of S2. We will measure the absolute rate of synthesis of S2, we will test for the existence of precursor forms of the protein by kinetic methods, and we will determine the polysomal stoichiometry of S2.